Multi-beam heterodyne laser Doppler vibrometer

ABSTRACT

A multi-beam laser Doppler vibrometer simultaneously measures velocity, displacement, and vibration history of multiple locations on an object. A beam of coherent light is split into an object beam and a reference beam. The object beam is divided into a plurality of object beams to simultaneously illuminate multiple locations on the object under inspection. The reference beam is frequency shifted and split into a corresponding plurality of frequency-shifted reference beams. A portion of each object beam is reflected by the object as a modulated object beam. The plurality of modulated object beams are collected and respectively mixed with the plurality of frequency-shifted reference beams to provide a plurality of beam pairs. Each beam pair may be focused onto a photodetector or an optical fiber connected to a photodetector.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 10/405,045 filed Mar. 31, 2003, which has issued asU.S. Pat. No. 6,972,846.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Contract NumbersDAAB07-00-C-F602 and DAAB07-01-C-L853 awarded by the U.S. Department ofDefense to MetroLaser, Inc. The government has certain rights in theinvention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to the measurement of vibration usingnon-invasive, non-contact, and remote techniques; namely, multiple beamsof coherent radiation are used as a probe to simultaneously measurevibrations at multiple locations on an object.

2. Description of the Related Art

Laser Doppler vibrometry (LDV) is a well-known non-contact method tomeasure the vibration of an object. Fields of application include:automotive, aerospace, and civil engineering; landmine detection;non-destructive testing; and non-contact sensing. LDV techniques arebased on the use of an interferometer to measure the Doppler frequencyshift of light scattered by a moving object. The motion of the objectrelative to the light source causes a shift of the light's frequency asdescribed by the Doppler equations.

There are two interferometric methods conventionally used for LDVapplications: homodyne detection and heterodyne detection. An opticalquadrature homodyne interferometer is a simple design utilizinglow-frequency photodetectors and amplifiers. However, the non-linearbehavior of these components causes harmonic distortions of the measuredsignal and an overall reduction in accuracy.

The heterodyne detection method using frequency shifting techniquesovercomes a number of drawbacks inherent in homodyne detection,including: (a) harmonic doubling that occurs when a source is located amultiple number of wavelengths away from the target; (b) non-linearitythat occurs at vibration amplitudes on the order of the measurementradiation's wavelength; (c) a low signal-to-noise ratio caused bysensitivity to laser intensity fluctuations; and (d) inverse frequency(i.e., l/f) detector noise. Both homodyne and heterodyne LDV systemsbased on single-point measurement techniques have been extensivelyinvestigated and form the basis of various conventional commercialinstruments.

Devices consisting of a single-beam LDV system in concert with a beamscanning system have also been developed. Scanned single-beam techniquesare suitable for measuring vibrations that are repetitive (e.g.,continuously cycling over the same location); however, because themeasurements are made sequentially from one location to the next, thevalue of this technique is limited when the vibrations are transient ornon-repetitive. Measurement of non-repetitive vibrations is importantwhen analyzing civil structures, aerospace composite components, andgolf clubs, as well as for buried land mine detection. While a pluralityof single-beam LDV systems could be used to measure multiple locationson an object, this would be a costly and complicated option if a largenumber of simultaneous measurements were required.

Simultaneous measurement of multiple locations on an object is needed inorder to gain more complete data on an object's vibrationalcharacteristics. Specifically, simultaneous LDV measurements yield: (a)phase information among the measured points, (b) increased inspectionspeed, and (c) the ability to measure non-repetitive vibration patterns.A simultaneous multi-beam LDV system based on a homodyne interferometerdesign has also been investigated. However, because that multi-beamtechnique is based on a homodyne detection method, it is affected by thesame performance limitations as the single-beam homodyne systemdescribed above.

In view of the foregoing, there is a need in the art for an LDV devicethat can simultaneously measure multiple locations on an object with thebenefits of high signal-to-noise ratio, wide dynamic range, and highaccuracy inherent with heterodyne detection.

BRIEF SUMMARY OF THE INVENTION

A heterodyne multiple-beam laser Doppler vibrometer (MBLDV)simultaneously measures displacement or velocity history of a multitudeof locations on an object or multiple objects. Simultaneous heterodynemeasurement of multiple locations provides a highly accurate measurementof an object's vibrational characteristics, especially transientvibrations. Such precise measurement of non-repetitive vibrationsenhances the capability of laser Doppler velocimetry.

According to one aspect of the invention, a multi-beam heterodynevibrometer includes an optical system and a combining element. Theoptical system generates a plurality of object beams and a plurality ofreference beams. A frequency shift is applied to either the object beamsor the reference beams (or both) so that the reference beams have afrequency that is shifted from a frequency of the plurality of objectbeams. The object beams are then transmitted to an object. A portion ofeach of the object beams is scattered and reflected off of the object asa modulated object beam. The modulated object beams are then collectedby the optical system. The combining element combines each of themodulated object beams with a respective one of the reference beams intoa plurality of beam pairs. The beam pairs may then be processed todetermine characteristics of the object.

One of the advantages of the invention is that the vibrometer is able tosimultaneous measure velocity or displacement of an object over multiplepoints. The heterodyne technique utilized by the vibrometer enablesmeasurements near zero frequency with excellent fidelity. Measurementsmade by the vibrometer of the invention are characterized by highsignal-to-noise ratio, wide dynamic range, and simple alignment. Thesystem may utilize a computer with software for computing and displayingthe velocity and/or amplitude history of all of the measured points ofthe object.

Other features and advantages of the present invention will becomeapparent to those skilled in the art from a consideration of thefollowing detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF SEVERAL VIEW OF THE DRAWINGS

FIG. 1 is a block diagram of a multi-beam laser Doppler vibrometer(MBLDV) system of the invention;

FIG. 2 is a schematic view of a MBLDV implemented in an object-detectionembodiment;

FIG. 3 is a flow chart illustrating methodology according to a number ofembodiments of the invention;

FIG. 4 is a schematic view of a MBLDV according to some of theembodiments;

FIG. 5 is a schematic view of an embodiment of an analyzing portion ofthe MBLDV system;

FIG. 6 is a schematic view of another embodiment of the analyzingportion;

FIG. 7 is a block diagram that schematically illustrate generalprinciples of the invention;

FIGS. 8A and 8B schematically illustrate beam-splitting principles ofthe invention according to a number of embodiments;

FIGS. 9A and 9B schematically illustrate beam-splitting principles ofthe invention according to other embodiments;

FIGS. 10A and 10B schematically illustrate principles for directing abeam onto a beam splitter according to still other embodiments;

FIG. 11 is a schematic view of an embodiment of a collecting andcombining portion of the invention;

FIG. 12 is a schematic view of another embodiment of the collecting andcombining portion of the invention;

FIG. 13 illustrates an optical relationship between a focusing lens andan output element of the invention; and

FIG. 14 is a schematic view of a MBLDV implemented in a scanningembodiment.

DETAILED DESCRIPTION OF THE INVENTION

A multiple-beam laser Doppler vibrometer (MBLDV) of the inventionsimultaneously measures the vibrational characteristics at multiplelocations on a target object. The MBLDV may be utilized in a number ofdifferent applications. For example, in a number of embodiments theMBLDV may be used in concert with acoustic techniques to measuretransient vibrations in a land mine detection system. In otherembodiments the MBLDV may be used to find internal flaws in structuralcomponents (e.g., composite materials, honeycomb, etc.). The MBLDV mayalso be used with or without additional acoustic excitation to measurevibration on automotive components, vibration on aircraft components,vibration on musical instruments, vibration on office machines (e.g.,copiers and computers). In other embodiments the MBLDV may be used tomeasure vibration from bridges and civil engineering structures.

Referring to the drawings in more detail, a block diagram of anexemplary embodiment of a MBLDV system 9 is illustrated in FIG. 1.Multiple beams T (shown in solid line) are transmitted to and illuminatean object 10 under inspection, and back-scattered radiation M (shown indashed line) from the beams T is collected by an optical subsystem 11.In a number of embodiments, the optical system processes theback-scattered radiation M and provides data associated with theradiation M to an electronics subsystem 12.

Depending upon the application, the electronics subsystem 12 may furtherprocess the data prior to providing the data to an analysis subsystem13. After analysis, the data may be provided to a data output subsystem14. In some of the embodiments, subsystems 11, 12, 13, and 14 may beincorporated as a single-unit MBLDV 15. The data output subsystem 14 ofthe MBLDV 15 may provide processed data to a data collection and displaysystem 16 of the system 9.

According to a number of embodiments, the MBLDV system 9 may beconfigured as a buried land mine detection system, which is depicted inFIG. 2. In these embodiments, the system 9 may include the MBLDV 15 andan audio source 17 such as a speaker. Acoustic vibrations V from theaudio source 17 send vibrations into the ground G. When coupled to aburied object 10 a, vibrations B modulate the surface S of the soil. Thetransient vibrations of the soil may then be measured by the MBLDV 15 todetect the object 10 a. In this embodiment, the transmitted object beamsTs are scattered and reflected by the soil S, not the object 10 anecessarily.

The system 9 illustrated in FIG. 2 may also be utilized in anondestructive testing embodiment. In these embodiments, the object 10 amay be a subsurface flaw, which flaw is detected by the MBLDV 15 afterexcitation from an acoustic source 17. In other embodiments, the MBLDV15 may analyze self-excitation objects such as loud speakers andengines.

Referring to FIG. 3, in a number of embodiments the MBLDV system 9operationally may provide a coherent laser source 18 which is dividedinto an object beam and a reference beam at process 19. At process 20the object beam may be divided into a plurality of object beams, which,at process 21, may then be focused onto a targeted object 10. Vibrationsat multiple locations on the object 10 modulate the multiple objectbeams simultaneously. The modulated object beam radiation is scatteredand reflected back by the object 10, which is also know asbackscattering. At process 22, this reflected and scattered radiationfrom the object 10 may be collected by the optical subsystem 11 of theMBLDV 15.

Regarding the reference beam, at process 23 a frequency shift may beadded to the reference beam. In a number of embodiments, thefrequency-shifted reference beam may be referred to as a localoscillator. At process 24 the frequency-shifted reference beam may bedivided into a plurality of reference beams. At process 25 the modulatedreflected and scattered radiation from the object and the plurality offrequency-shifted reference beams may be combined and, at process 26,focused into a plurality of fiber optics or detectors.

At process 27 data collected by the detectors may be analyzed. Forexample, surface displacement of each of the object beams may becalculated using a computer, displayed on a monitor, or output to aperipheral device of the display and collection system 16.

In alternative embodiments, rather than utilizing a frequency-shiftedreference beam, the MBLDV 15 may be configured to frequency shift theobject beam. The resulting frequency-shifted object beam may be splitinto a plurality of frequency-shifted object beams that are transmittedto the object. In this regard, the optical beam path in FIG. 3 mayinclude process 23 in which a frequency shift is added to the objectbeam prior to process 20 in which the beam is divided.

With reference to FIG. 4, according to a number of embodiments, theoptical sub-system 11 may include a laser 18 as a radiation source. Forexample, the laser 18 may be a single continuous-wavefrequency-stabilized diode-pumped Nd:YAG laser that produces anultra-narrow line-width 532 nm beam with a single axial mode.

The output beam L of the laser 18 is reflected by a mirror 28 a. A firsthalf-waveplate (HWP) 29 may be positioned downstream of the mirror 28 ato rotate the polarization of the beam and thus provide variable controlof the irradiance of beams downstream thereof. The output beam L maythen be split into an object beam O and a reference beam R at a firstpolarizing beam splitter (PBS) 30. As shown in the example in FIG. 4,the S-polarized portion of the output beam L is reflected by the PBS 30and used as the object beam O, and the P-polarized portion of the outputbeam L is transmitted by the PBS 30 and used as the reference beam R.(For purposes of this description, the conventional scientific notationfor polarization is used in the drawings, i.e., an arrow forP-polarization and a dot for S-polarization.)

Regarding a path of the object beam O (i.e., the object path), a secondHWP 31 may be positioned downstream of the PBS 30 to rotate the objectbeam O to P-polarization. A telescope formed by a first lens 32 and asecond lens 33 may be utilized to expand the object beam O from wherethe object beam O is incident at a first (or object) diffractive opticalelement (DOE) 34, which splits the object beam O into a plurality ofobject beams Os. The lenses 32 and 33 may also focus the object beams Osonto the targeted object 10. The plurality of object beams Os may thenbe transmitted through a second PBS 35 and a quarter-waveplate (QWP) 36and then transmitted to the object 10 under inspection, whichtransmitted object beams are indicated by Ts. The plurality oftransmitted object beams Ts irradiate substantially simultaneously acorresponding plurality of points on the object 10.

In alternative embodiments, a plurality of lasers, beam splitters, andother optical element may be used to split an object beam into aplurality of object beams. Furthermore, the pattern of the object beamsmay follow different geometries depending on the choice of beamsplitter. For example, in the embodiment shown in FIG. 4, the objectbeam O is linearly split into a plurality of object beams Os. In otherembodiments, the object beam O may be split into a plurality of objectbeams that form a matrix of X columns and Y rows. In still otherembodiments, the optical subsystem 11 may include arbitrary multiplebeam patterns and scanning of the beams.

Vibrations of the object 10 frequency modulate each of the transmittedobject beams T substantially simultaneously. A portion of thesemodulated object beams, which are indicated by Ms, is scattered andreflected back to the optical subsystem 11. In the embodiment shown inFIG. 4., the modulated object beams Ms may pass through the QWP 36 androtate to S-polarization. The modulated object beams Ms may then bereflected at the PBS 35 and collected by a lens 37. Downstream of lens37 may be positioned a beam combiner 38, a lens 39, an optical output40, and a sensing element 41, which will be discussed in more detailbelow.

Turning to a path of the reference beam R (i.e., the reference path), inthe embodiment shown in FIG. 4 the reference beam R may be initiallyfrequency shifted. To do so, in a number of embodiments the referencebeam R may travel sequentially through one or more acousto-opticmodulators. In the embodiment shown, two acousto-optic modulators (AOM)42 and 43 are employed. In some of the embodiments, the AOMs 42 and 43may be operated at different frequencies, for example, frequency f₁ andfrequency f₂, respectively, so that different diffraction orders of thereference beam R may be selected. For example, the first AOM 42operating at frequency f₁ may select the +1^(st) order diffracted beamof the reference beam R, and the second AOM 43 operating at frequency f₂may select the −1^(st) order diffracted beam of the reference beam R. Inthis example, the reference beam R is frequency shifted by thedifference of frequencies f₁ and f₂. As an example, AOM 42 operating ata frequency of 80 MHz and AOM 43 operating at a frequency of 80.1 MHzproduce a frequency shift of 100 kHz on the reference beam R.

In other embodiments of the invention, the object beam O can befrequency shifted to achieve a carrier frequency that yields a typicalheterodyne signal in detectors of a detector array. As in the case ofthe reference beam, this frequency shift may be obtained with a singleAOM, two AOMs, or various other devices such as rotating gratings,micro-electromechanical system (MEMS), liquid crystal gratings, andother devices to shift the frequency of the beams.

Upon being frequency shifted, the reference beam R may be reflected by amirror 28 b through a third HWP 44 to rotate the reference beam R toS-polarization so that the reference beam R has the same polarization asthe collected modulated object beams Ms after they pass through thequarter waveplate 36 and beamplitter 35. The reference beam R may thenbe expanded and collimated by a telescope formed by a first lens 45 anda second lens 46.

The collimated reference beam R may then be reflected from a mirror 28 cthrough a second (or reference) DOE 47. In a number of embodiments, thereference DOE 47 may be identical to the object DOE 34. The referenceDOE 47 splits the reference beam R into a plurality of reference beams.In embodiments in which the DOEs 34 and 47 are substantially the same,the number of object beams is equal to the number of reference beams(e.g., 16); in addition, the angle at which the object beams divergefrom the object DOE 34 is substantially equal to the angle at which thereference beams diverge from the reference DOE 47. For example, the DOEs34 and 47 may split the beams O and R into 16 beams Os and Rs,respectively, which beams diverge from the DOEs at an angle of about 22degrees. Those skilled in the art will appreciate that the DOEs 34 and47 may be modified to produce any number of beams that diverge at anyangle.

Upon splitting, the reference beams Rs may be reflected from a mirror 28d toward the beam combiner 38. In accordance with a number ofembodiments, the beam combiner 38 combines each of the plurality ofreference beams Rs with a respective one of the modulated object beamsMs, thereby yielding a corresponding plurality of beam pairs Ps, whereineach beam pair P includes a modulated object beam M and afrequency-shifted reference beam R. In some of the embodiments, the beamcombiner 38 aligns each of the reference beams Rs to overlap with one ofthe modulated object beams Ms.

The plurality of beam pairs Ps may then be focused on an optical output,such as a plurality of optical fibers 40, which may be coupled to adetecting element, such as a plurality of photodetectors 41 located inthe electronics subsystem 12. Lenses 37 and 39 focus or image themodulated object beams Ms onto the optical fibers 40. In otherembodiments the plurality of photodetectors may replace the opticaloutput so that the beam pairs Ps focus directly on the photodetectors.In other embodiments the detecting element may include coherent fiberbundles, photodiodes, CMOS detector arrays, or CCD detector arrays.

Each of the detectors 41 senses a frequency-modulated signal of a beampair P. The frequency-modulated signal of each beam pair P has a carrierfrequency f_(c) given by the difference of the frequencies of the AOMs42 and 43, namely, f_(c)=f₁−f₂. In addition, frequency-modulated signalof each beam pair P has a frequency deviation caused by vibration of theobject 10.

In a number of embodiments, in addition to providing an interface forthe signals output by the optical subsystem 11, the electronicssubsystem 12 may provide power to and filtering functions for thedetectors 41. Further, the electronics subsystem 12 may be coupled tothe AOM 42 and 43 to provide power and drive signals to the AOMs forfrequency shifting the reference beam R.

As mentioned, each optical fiber 40 of the optical subsystem 11 may becoupled to a photodetector 41 of the electronic subsystem 12. Each ofthe detectors 41 has a bandwidth that may be chosen to be larger thanthe carrier frequency f_(c) of the beam pairs Ps. For example, iffrequency f₁ of AOM 42 is 80.0 MHz and frequency f₂ of AOM 43 is 80.1MHz, then the bandwidth of the detectors 41 may be larger than 100 kHz(i.e., f₁−f₂), for example, about 1 MHz.

Referring to FIG. 5, in a number of embodiments, the electronicssubsystem 12 may include a reference clock 48 for generating the carrierfrequency f_(c). In some of the embodiments, the clock 48 may include apair of radio frequency (RF) drivers 49 and 50 respectively operating atfrequency f₁ and frequency f₂, which frequencies may be provided to theAOMs 42 and 43, respectively. An electronic mixer 51 may mix frequenciesf₁ and f₂ to generate a frequency equal to the sum (f₁+f₂) and afrequency equal to the difference (f₁−f₂). A low-pass filter 52 may bepositioned downstream of the mixer 51 to eliminate the sum frequency,thereby providing a carrier signal C at the frequency f_(c).

The electronics subsystem 12 may also include a pair of bandpass filters53 and 54 each of which is centered at the carrier frequency f_(c)(i.e., f₁−f₂). The filters 53 and 54 maintain a high signal-to-noiseratio in frequency-modulated signals Fs output from the detectors 41 andthe carrier signal C output from the reference clock 48. In thoseembodiments in which a plurality of optical fibers 40 respectivelyreceive the plurality of beam pairs Ps (see FIG. 4), each of thefrequency-modulated signals Fs corresponds to a respective one of thebeam pairs Ps and, accordingly, to a respective one of the modulateobject beams Ms.

The carrier signal C from the reference clock 48 and thefrequency-modulated signals Fs from the detectors 41 may then bedigitized by a simultaneous sampling analog-to-digital (A-D)data-acquisition system 55. In a number of embodiments, the samplingrate of the A-D system 55 may be sufficiently large when compared to thecarrier frequency f_(c); for example, if the carrier frequency f_(c) isabout 100 kHz, then the sampling rate may be at least about 300 kHz.

When digitized, the frequency-modulate signals Fs and the carrier signalC may be provided to the analysis subsystem 13. In a number ofembodiments, the analysis subsystem 13 utilizes software to analyze thesignals Fs from the detectors 41. In a number of embodiments, thefrequency-modulated signals Fs and the carrier signal C may be initiallycleaned with one or more digital bandpass filters 56. The carrier signalC may then be phase-shifted 57, for example, by 90 degrees, with asoftware algorithm, such as a digital Hilbert transform.

The carrier signal C and the phase-shifted carrier signal may then bemixed with each of the frequency-modulated signals Fs, which isindicated at 58 and 59, respectively. Each of the mixed signals may thenbe low-pass filtered 60 and 61. The result of this digital processingare a set of in-phase signals I and a set of quadrature signals Qsignals for each of the frequency-modulated signals Fs.

The in-phase and quadrature signals I and Q may then be processed by aprocessor 62 to determine a number of attributes of the object 10. Forexample, in a number of embodiments object displacement may becalculated utilizing the inverse tangent of the in-phase and quadraturesignals I and Q. In addition, discontinuities at steps of 2π may beremoved from signals I and Q by standard phase unwrapping techniques.Alternatively, object velocity may be calculated from the quantity:[(I′×Q)−(I×Q′)]÷[I²+Q²].

In a number of embodiments, when the displacement (or the velocity) ofthe object 10 has been calculated as a function of time for each of thefrequency-modulated signals Fs, then signals I and Q may be Fourieranalyzed to determine the displacement (or the velocity) as a functionof frequency. The resulting Fourier spectra may then sent by a computer63 of the display subsystem 16 to an output device, such as a monitor64. Alternatively, the computer 63 may provide a hardcopy of theanalyzed data to a printer 65 or transmit the data to a remote location66, for example, over a network. A software package may be implementedon the computer 63 for processing data associated with thefrequency-modulated signals Fs.

In a number of embodiments, the Fourier spectra may be displayed in a“waterfall”-type graph with a small offset applied to each spectrum sothat all of the spectra can be viewed simultaneously. In addition, aone-dimensional or a two-dimensional surface map of the displacement (orthe velocity) of the object 10 may be created at a given frequency band.To improve the processing speed of the analysis subsystem 13,displacement-versus-time signals can be safely down-sampled from thehigh sampling frequency (e.g., at least about 300 kHz) down to a lowsampling frequency (e.g., less than 10 kHz), thereby resulting in fasterFourier transform calculations. Overall, each of the heterodyne signalsFs corresponding to each of the modulated object beams Ms may besimultaneously analyzed with one or more demodulation techniques.

Alternative embodiments of the electronics subsystem 12 and the analysissubsystem 13 are illustrated in FIG. 6. To maintain a highsignal-to-noise ratio, the electronics subsystem 12 may include apre-amplifier stage and an amplifier stage 67 to amplify thefrequency-modulated signals Fs output from the detectors 41. Inembodiments both analog and digital processing is desired, thefrequency-modulated signals Fs may be provided to an analog output 68through phase-locked loop (PLL) circuitry 69. Further, in addition todigitizing the frequency-modulated signals Fs and the carrier signal Cwith the A-D converter 55 for downstream conditioning by elements 56 to61, the frequency-modulated signals Fs may be digitized by a second A-Dconverter 70, the output of which is fed directly to the processor 62.

With further reference to FIG. 2, according to a number of embodimentsthe vibrometer 15 may be configured to detect land mines. As mentionedabove, a loudspeaker 17 may be utilized to generate acoustic waves V inthe air, which couple into the ground G and cause the buried object 10(i.e., a land mine) to vibrate. The vibrations of the object 10(indicated by B) cause the soil S above the object 10 to vibrate. TheMBLDV 15 may then be utilized to collect the modulated object beams Msand analyze the resulting frequency-modulated signals Fs. For example,the velocity of the soil S at multiple points may be calculated. Thepresence of a buried object 10 can be determined by studying the spatialdistribution of the soil velocity spectra.

Because of the plurality of transmitted object beams Ts, the MBLDV 15 isable to simultaneously measure a plurality of points, therebysubstantially reducing the time required take the measurements necessaryto detect a buried object 10. For example, if there are 16 transmittedobject beams Ts, then the MBLDV 15 would reduce the necessarymeasurement time by a factor of 16 over conventional single-beamscanning systems. A one-dimensional beam array may then be translated inthe cross-dimension, thereby providing coverage of a two-dimensionalarea in a much shorter time than a conventional scanning method.

According to a number of embodiments, the MBLDV 15 may be configured todetect both surface defects (e.g., cracks) and subsurface defects (e.g.,disbonds, delaminations, etc.) in aircraft and ship panels. In theseembodiments, the surface of a panel may be excited by an acoustic ormechanical source. The resulting surface displacement (or velocity)profile may then be examined with by the MBLDV system 9. The presence ofa surface defect, such as a crack, causes a change in the vibrationalpattern of the surface. The presence of a subsurface defect may cause anenhanced velocity above the defect at a particular resonance frequency.In contrast to conventional scanning devices where the surface must beexcited repetitively, the MBLDV 15 of the invention is able to analyze aplurality of points simultaneously with a single excitation of thesurface. In addition, the MBLDV 15 is able to provide the relative phasebetween each analyzed point so that modal vibrational patterns can beimmediately observed.

In addition to detecting defects as described above, the MBLDV 15 hasmany other applications. For example, the vibration pattern of a testobject may be measured, such as speakers, musical instruments,automotive disc brakes, computer disks, car panels, airplane panels, andso on. In these embodiments, the MBLDV 15 measures the vibration of theobject without any additional acoustic excitation.

Turning to a more general description of the principles of the inventionwith reference to FIG. 7, a multi-beam heterodyne vibrometer 100 foranalyzing vibration of an object 10 includes an optical system 102 and acombining element 104. In a number of embodiments, the optical system102 generates a plurality of object beams T that are transmitted to theobject 10 and a plurality of frequency-shifted reference beams R. Theplurality of frequency-shifted reference beams R having a frequency thatis shifted from a frequency of the plurality of transmitted object beamsT. A portion of each of the transmitted object beams T is reflected offof the object 10, thereby producing a plurality of modulated object beamM. The optical system 102 also collects the modulated object beams Mreflected by the object 10. The combining element 104 combines each ofthe modulated object beams M with a respective one of thefrequency-shifted reference beams R into a plurality of beam pairs P,which may be provided to a process stage 106.

Referencing FIGS. 8A and 8B, the optical system 102 includes a pair ofbeam splitters, such as the DOEs 34 and 47 (see FIG. 4), forrespectively splitting the object beam O into the plurality of objectbeams Os and the frequency shifted reference beam R into the pluralityof frequency-shifted reference beams Rs. DOE 34 is configured to splitthe object beam O so that the plurality of object beams Os diverge froman optical axis A of the DOE 34 at a divergence angle ω. Similarly, DOE47 is configured to split the frequency-shifted reference beam R so thatthe plurality of frequency-shifted reference beams Rs diverge from theoptical axis A of DOE 47 at a divergence angle ρ. The divergence of theobject beams Os and the frequency-shifted reference beams Rs is alsoshown in FIGS. 9A and 9B. According to a number of embodiments, the DOEs34 and 47 are configured so that divergence angle ω of the object beamsOs is substantially the same as the divergence angle ρ of thefrequency-shifted reference beams Rs.

With continued reference to FIGS. 9A and 9B, in other embodiments theDOEs 34 and 47 may be configured to split the object beam O and thefrequency-shifted reference beam R, respectively, so that the number ofobject beams Os is equal to the number of frequency-shifted referencebeams Rs. For example, the number of beams may be an exponential of 2,i.e., 2^(n) where n is an integer.

With additional reference to FIGS. 10A and 10B, the optical system 102may also include a first telescope 108 for focusing or directing theobject beam O onto one of the beam splitters, i.e., HOE 34, and a secondtelescope 110 for focusing or directing the frequency-shifted referencebeam R onto the other one of the beam splitters, i.e., DOE 47. As shownin the drawings, in a number of embodiments, the first telescope 108 mayinclude lenses 32 and 33 and the second telescope 110 may include lenses45 and 46 of the embodiment illustrated in FIG. 4. As mentioned above,the first telescope 108 also focuses the plurality of object beams Osonto the object 10.

As shown in FIGS. 10A and 10B, the object beam O has a diameter d_(O),and the frequency-shifted reference beam R has a diameter d_(R).According to a number of embodiments, the telescopes 108 and 110 areconfigured to direct or closely collimate the object beam O and thefrequency-shifted reference beam R, respectively, onto a respective oneof the beam splitters, i.e., DOEs 34 and 47, respectively, such that thediameters of the beams O and R are substantially equal when incidentupon the DOEs 34 and 47. The diameters of the object beam O and thefrequency-shifted reference beam R when incident with the DOEs 34 and 47are indicated by D_(O) and D_(R), respectively. For example, the firsttelescope 108 may slightly focus the object beam O onto DOE 34 so that abeam diameter D_(O) is produced at DOE 34, while sharply focusing thetransmitted object beams Os at the target 10. The second telescope 110may collimate the frequency-shifted reference beam R onto DOE 47 so thatdiameters D_(O) and D_(R) are substantially the same.

With reference to FIG. 11, the optical system 100 may include acollecting lens 37 for collecting the modulated object beams Ms. Thecollecting lens 37 has a focal length f₃₇. According to a number ofembodiments, the optical system 100 is positioned from the object 10 bya distance substantially equal to the focal length f₃₇ of the collectinglens 37. Accordingly, the modulated object beams Ms are individuallycollimated by the collecting lens 37 while maintaining the divergenceangle ω as the beams Ms travel through the center of the lens 37.

The optical system 100 may also include a focusing lens 39 positioneddownstream from the combining element 38 and having a focal length f₃₉.In a number of embodiments, the focusing lens 39 is positioned from thecollecting lens 37 by a distance substantially equal to focal lengthf₃₉. In other embodiments, an output element 111, such as a detectorelement, may be positioned downstream from the focusing lens 39 by adistance substantially equal to focal length f₃₉.

In still other embodiments, the DOE 47 for splitting thefrequency-shifted reference beam R is positioned upstream from thefocusing lens 39 by a distance substantially equal to focal length f₃₉.For example, as shown in FIG. 11, the sum of a distance l₁ from DOE 47to the combining element 38 and a distance l₂ from the combining element38 to the focusing lens 39 is substantially equal to focal length f₃₉.

Referring to FIG. 12, the optical system 100 may also includes anaperture 112 positioned adjacent to the collecting lens 37. The aperture112 has a diameter d_(AO). In addition, the frequency-shifted referencebeams Rs each have a diameter d_(R). According to a number ofembodiments, the diameter d_(AO) of the aperture 112 is substantiallyequal to the diameter d_(R) of the frequency-shifted reference beams Rs.As a result, a diameter d_(M) of the modulated object beams Ms issubstantially equal to the diameter d_(R) of the frequency-shiftedreference beams Rs.

In other embodiments, the optical system 100 may include a secondaperture 114 positioned at an upstream side of DOE 47. Thereference-path aperture 114 has a diameter d_(AR). In a number ofembodiments, the diameter d_(AR) of the reference-path aperture 114 issubstantially equal to the diameter d_(AO) of the object-path aperture112.

As mentioned above, in a number of embodiments the polarization of themodulated object beams Ms and the polarization of the frequency-shiftedreference beams Rs are the same. As shown in FIG. 12, the polarizationsare indicated as S-polarization with the dots D.

In addition to collecting the modulated object beams Ms, the collectinglens 37 may also collimate the modulated object beams Ms. The collimatedmodulated object beams are indicated by Ms′ in FIG. 12. Similarly, theplurality of frequency-shifted reference beams Rs may be collimated bythe telescope 110 lens prior to being combined into the beam pairs Ps.

Furthermore, the modulated object beams Ms′ diverge from an optical axisA of the collecting lens 37 at an angle μ. According to a number ofembodiments, the divergence angle μ of the modulated object beams Ms′ issubstantially equal to the divergence angle ρ of the frequency-shiftedreference beams Rs from DOE 47. Also as shown in FIG. 12, the focusinglens 39 may render the plurality of beam pairs Ps substantially parallelwith each other, which parallel beam pairs are indicated by Ps′.

With additional reference to FIG. 13, the focusing lens 39 may bepositioned a focal length f₃₉ away the output element 111 so that lens39 focuses each of the parallel beam pairs Ps′ onto the element. Asmentioned above, the output element 111 may be an array including aplurality of elements 115, such as photodetectors or optical fibers.Accordingly, the focusing lens 39 focuses each of the beam pairs Ps′onto a respect one of the elements 115. In embodiments where theelements 115 are optical fibers, a plurality of photodetectors may beprovided downstream from the optical fibers. In either embodiment, theoutput element 111 converts the optical signals of the plurality of beampairs Ps into the frequency-modulated signals Fs (either analog ordigital signals) mentioned above.

The MBLDV 15 of the invention may also be implemented in a scanningmulti-beam system 116 as shown in FIG. 14. According to scanningembodiments, the system 116 may include a mirror 118 that is rotatableabout an axis A under actuation by a motor 120. The plurality of objectbeams Os transmitted by the MBLDV 15 may then be reflected off of themirror 118 simultaneously and onto an object 10 under inspection, asindicated by Os′. The modulated object beams Ms may then be collected bythe MBLDV 15. The mirror 118 may then be rotated so that the reflectedobject beams Os′ are incident at a different location of the object 10.Accordingly, over time a plurality of points t₁, t₂, t₃, t₄, . . . areanalyzed by each of the object beams Os′.

One of the advantages of the scanning MBLDV 116 is that the mirror 118only needs to rotated about a single axis A. Although a two-axisrotatable mirror may be utilized, the single-axis mirror 118 is able toscan a surface of the object 10. More specifically, the MBLDV 15provides a matrix of a plurality of object beams Os, e.g., an X matrix.The entire matrix may then be scanned in one axis, e.g., a Y matrix) toprovide an X-Y matrix of data. Accordingly, over a relatively shortperiod of time, an entire surface of the object 10 may be scanned by thematrix of object beams and analyzed.

For the purposes of the following claims, the phenomenon of thetransmitted object beams Os being reflected, scattered, andbackscattered by the object 10, thereby resulting in the modulatedobject beams Ms, will be simply indicated by the word reflect and itsforms, such as reflected.

Those skilled in the art will understand that the preceding exemplaryembodiments of the present invention provide the foundation for numerousalternatives and modifications thereto. For example, although thepreceding description set for the principles of the invention in termsof a coherent light source such as a laser, other types of coherentradiation may be utilized, such as microwaves, millimeter waves, and soon. These and other modifications are also within the scope of thepresent invention. Accordingly, the present invention is not limited tothat precisely as shown and described above but by the scope of theappended claims.

1. A multi-beam heterodyne vibrometer for analyzing vibration of anobject, the vibrometer comprising: an optical system for: generating aplurality of object beams and a plurality of reference beams, one of theplurality of beams having a frequency that is shifted from a frequencyof the other plurality of beams; transmitting the object beams to theobject, a portion of each of the object beams being reflected off of theobject as a modulated object beam; and collecting the modulated objectbeams; and a combining element for combining each of the modulatedobject beams with a respective one of the reference beams into aplurality of beam pairs.
 2. The vibrometer of claim 1 wherein theplurality of object beams is frequency shifted.
 3. The vibrometer ofclaim 1 wherein the plurality of reference beams is frequency shifted.4. The vibrometer of claim 1 wherein the plurality of modulated objectbeams and the plurality of reference beams have the same polarization.5. The vibrometer of claim 1 wherein the plurality of modulated objectbeams and the plurality of reference beams are collimated prior to beingcombined into the beam pairs.
 6. The vibrometer of claim 1 wherein theoptical system includes a pair of beam splitters for respectivelysplitting an object beam into the plurality of object beams and areference beam into the plurality of reference beams.
 7. The vibrometerof claim 6 wherein the beam splitters split the object beam and thereference beam such that the plurality of object beams and the pluralityof reference beams diverge from the respective beam splitter atsubstantially the same angle.
 8. The vibrometer of claim 6 wherein theoptical system further includes: a laser for providing an output beam;and an output beam splitter for splitting the output beam into theobject beam and a reference beam; one of the beams being frequencyshifted prior to being split into the plurality of frequency-shiftedbeams.
 9. The vibrometer of claim 1 wherein the optical system includesa collecting lens for collecting the modulated object beams and anaperture positioned next to the collecting lens; the collecting lenshaving a focal length and being positioned from a surface of the objectby a distance substantially equal to the focal length.
 10. Thevibrometer of claim 9 further comprising a focusing lens positioneddownstream from the combining element and having a focal length, whereinthe focusing lens renders the plurality of beam pairs substantiallyparallel with each other.
 11. The vibrometer of claim 10 furthercomprising an output element including a plurality of optical fibersconnected to a plurality of photodetectors being positioned downstreamfrom the focusing lens by a distance substantially equal to the focallength of the focusing lens, the focusing lens for focusing each of thebeam pairs onto a respective one of the optical fibers.
 12. Thevibrometer of claim 1 further comprising a mirror for reflecting thetransmitted object beams onto the object and a motor for rotating themirror so that the transmitted object beams scan the object.
 13. Amethod for analyzing vibration of an object, the method comprising:transmitting a plurality of object beams to the object, a portion of theobject beams being reflected by the object as modulated object beams;collecting the modulated object beams; and combining the modulatedobject beams with a reference beam; wherein one of the combined beamshas a frequency that is shifted from a frequency of the other of thecombined beams.
 14. The method of claim 13 wherein the plurality ofobject beams is frequency shifted.
 15. The method of claim 13 whereinthe reference beam is frequency shifted.
 16. The method of claim 13further comprising splitting an object beam into the plurality of objectbeams.
 17. The method of claim 16 wherein the object beam has afrequency that is shifted from the frequency of the reference beam. 18.The method of claim 16 wherein the reference beam has a frequency thatis shifted from the frequency of the object beam.
 19. The method ofclaim 13 further comprising splitting a reference beam into a pluralityof reference beams for respectively combining with the object beams. 20.The method of claim 13 further comprising splitting a coherent lightbeam into the object beam and the reference beam.
 21. The method ofclaim 13 wherein the object beam and the reference beam are respectivelysplit into an equal number of object beams and reference beams.
 22. Amulti-beam heterodyne vibrometer for analyzing vibration of an object,the vibrometer comprising: a laser configured to provide an output beam;a first beam splitter configured to split the output beam into an objectbeam and a reference beam; a frequency shifter configured to shift afrequency of either the object beam or the reference beam; a second beamsplitter configured to split the reference beam into a plurality ofreference beams; a third beam splitter configured to split the objectbeam into a plurality of object beams for transmission to the object, aportion of the object beams being reflected by the object as a modulatedobject beam; a collecting lens disposed to collect the plurality ofmodulated object beams; and a combining element for combining themodulated object beams with one of the reference beams into a pluralityof beam pairs.
 23. The vibrometer of claim 22 further comprising anoutput element for receiving the plurality of beam pairs.
 24. Thevibrometer of claim 23 wherein the output element is an array includinga plurality of elements.
 25. The vibrometer of claim 24 wherein theoutput element provides a plurality of frequency-modulated signals basedon the plurality of beam pairs.